The US has called for a war crimes investigation of Russia and Syria over the two countries' joint offensive in Aleppo.
The move by US secretary of state John Kerry ramps up the rhetoric against Moscow for its part in the conflict, while potentially making it harder to resume diplomatic efforts to end the fighting.
Mr Kerry said Syrian forces had hit a hospital outside Damascus overnight, killing 20 people and wounding 100.
Human rights group also accused the two countries of killing thousands in their assault on Aleppo, Syria's largest city.
Mr Kerry said: "Russia and the regime owe the world more than an explanation about why they keep hitting hospitals and medical facilities, and children and women.
"These are acts that beg for an appropriate investigation of war crimes.
"They're beyond the accidental now, way beyond, years beyond the accidental. This is a targeted strategy to terrorise civilians and to kill anybody and everybody who is in the way of their military objectives."
The Russian foreign ministry said Mr Kerry was trying to divert attention from America's failure to uphold a ceasefire in Syria.
"Kerry used these words from the point of view of fanning tensions," spokeswoman Maria Zakharova said.
"As long as war crimes are at question, the Americans should start with Iraq. And then look at Libya and Yemen to see what is going on there."
The US has little chance of being able to initiate a war crimes probe of either Russia or Syria. Russia has veto power at the UN Security Council and has blocked repeated attempts over the last five and a half years to put pressure on Syrian president Bashar Assad's government or hold it accountable for the widespread allegations of indiscriminate killing, torture and chemical weapons attacks.
French foreign minister Jean-Marc Ayrault spoke of a new French effort for a ceasefire in Syria that would include a UN Security Council vote on Saturday. But it is unclear what advantages his plan would have over the US-Russian led process that collapsed last month.
Mr Kerry's September 9 agreement with Russian foreign minister Sergey Lavrov would have created a new counter-terrorism alliance in Syria, had fighting stopped for a week and aid deliveries been permitted to reach desperate civilians in rebel-held parts of Aleppo and other besieged areas. Neither condition was ever met.
The truce then shattered completely when Syria and Russia renewed their military offensive in Aleppo. Mr Kerry ended bilateral discussions with Russia on the military partnership earlier this week.
Mr Ayrault called Syria a "human tragedy" that demands every effort to restart a peace negotiation.
He said Saturday would be a "moment of truth" at the Security Council. He said the question that will be posed to everyone, but particularly to Russia, is: "Do you, yes or no, want a ceasefire in Aleppo?"
Such a ceasefire would be "open to discussion", but Mr Ayrault said two demands were absolute.
"The first one is the ceasefire and no-fly zone over Aleppo," he said.
"And the second pillar is access for humanitarian aid. We're not giving up."
At the current rate of fighting, Mr Ayrault claimed, "Aleppo will be totally destroyed by Christmas".
Russia will almost surely veto the French measure.
"I cannot possibly see how we can let this resolution pass," Russia's UN ambassador Vitaly Churkin said.
The war has killed as many as half a million people since 2011, contributed to Europe's worst refugee crisis since the Second World War and allowed the Islamic State group (IS) to carve out territory for itself and emerge as a global terror threat.
Given the range of militant groups, extremists and outside powers now fighting in Syria, the original contest between Mr Assad's government and so-called "moderate" opposition forces has proven stubbornly difficult to quell.
Russia launched its air campaign in Syria a year ago, reversing the tide of war and helping Mr Assad's forces make significant territorial gains. Washington responded by engaging Moscow in a multinational process aimed at getting all of Syria's fighting parties, except IS and al-Qaida, to buy into a ceasefire and eventual unity government.
The US also wanted Russia to join in the campaign against extremist groups like IS.
For that reason, Mr Kerry and other US officials have tempered their criticism of Russia even as the death toll from its air campaign has risen. Syrian human rights observers claim Moscow has killed as many as 9,400 people, crushing hospitals, schools and other civilian infrastructure in rebel-held areas along the way.